USB: core: constify vm_operations_struct

vm_operations_struct are not supposed to change at runtime.
All functions working with const vm_operations_struct.
So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>